A crane has collapsed at a construction site in inner Melbourne, forcing the evacuation of the building and closure of nearby major roads.

The crane was in the process of jacking up to the next level when a hydraulic support gave way at the Southbank site near the corner of Kavanagh Street and Kings Way, according to the Metropolitan Fire Brigade (MFB).

The crane collapsed in on itself and was left resting on the lift shaft inside the building, about 50 metres above the ground.

One witness said he saw a wire snap moments before the crane crashed down.

Ambulance Victoria said one person was treated for minor leg injuries.

MFB commander George Arnold said the crane was not carrying concrete, despite earlier reports suggesting a slab had plummeted to the ground during the incident.

"We've isolated it, everyone's been evacuated out and we've set up a safety cordon," he said.

Mr Arnold said the crane was stable and experts would be brought in to work out how to get the crane upright.

He said a large evacuation zone was setup in the unlikely event of the crane collapsing.

The inbound Kings Way off-ramp was closed and traffic had to be diverted off at Lorimer Street.